1. Field of the Invention
The present invention relates to a method of making porous polymeric separation membrane for a lithium ion polymer battery and the lithium ion polymer battery containing the same, and more particularly to a method of making a polymeric porous separation membrane fabricated by gelling polymer and inorganic material on a porous polyolefin membrane and making a lithium ion polymer battery fabricated by using the above method of making a polymeric porous separation membrane having a good cycle life, a good retention of capacity at high rate discharge and an excellent characteristics for safety
2. Description of the Related Art
Recently, the electric, electronic, communication and computer industries have been rapidly developed, which has increased demand for secondary batteries that have high qualities and safeties. Especially, electric or electronic products tend to become smaller, thinner and more adequate to be carried and this tendency brings demand for smaller and thinner secondary batteries, the core component of the above electric or electronic products.
Currently, a lithium ion battery is used in a portable device and an electronic product. In the lithium ion battery, a porous film made of polyethylene or polypropylene is used as a separation membrane. To fabricate the lithium ion battery, electrodes and the separation membrane are wound and put in a cylinder shaped case or a rectangular shaped case, as it is difficult to flatly laminate the electrodes and the separation membrane. The rectangular shaped lithium ion battery fabricated by being wound as described above is commercially used but is complicated to fabricate and restricted by shape, so that it is limited for the battery to become thinner and to have more capacity. Thus, a lithium polymer battery is suggested as an alternative for the lithium ion battery; the lithium polymer battery is fabricated by a solid polymeric electrolyte available to be employed as the separation membrane and an electrolyte. Thus, according to kind of the electrolyte, the solid polymeric electrolyte and the electrodes can be not only flatly laminated but also be wound, thereby increasing productivity.
A conventional solid polymeric electrolyte is made usually of polyethyleneoxide, but it has very low conductivity of about 10−8 S(Siemens)/cm at normal temperature, so that it is not proper to be exclusively used. To solve the above problem, a polymeric electrolyte in a gel form was developed to have the conductivity of 10−3 S/cm. The representative polymeric electrolyte in a gel form is composed of polyacrylonitile disclosed in American U.S. Pat. No. 5,219,679, The gelled polymeric electrolyte disclosed in American U.S. Pat. No. 5,219,679 has a high conductivity and a strong cohesive force to the electrodes, but a low mechanical strength, so that it has a difficulty in being practically used.
A polymer compound of polyvinylideneflouride(PvdF) in a hybrid form was disclosed in American U.S. Pat. Nos. 5,296,319 and 5,460,904 by Gozdz, which attempted a mass-production of the lithium polymer battery in the hybrid form. However, to produce the above the lithium polymer battery, a plasticizer should be added to fabricate the solid polymeric electrolyte, an anode and a cathode, thereby causing a difficulty in fabricating the battery, as the plasticizer should be extracted later. Further, the PvdF electrolyte has a high mechanical strength, but weak cohesive force, so that the fabrication of the battery requires a process making layers thinner by heat at high temperature. Further, the electrodes and the gelled polymeric electrolyte are exfoliated in the extracting process, which causes a drop of quality of the battery.
Thus, the requirement to fabricate the battery by using the gel electrolyte cannot be accomplished by using exclusively the gel electrolyte having a low mechanical strength. To solve the above problem, Korean patent no. 2000-7004714 discloses a method to coat the electrodes with the gel electrolyte. However, the method to coat the electrodes with the gel electrolyte has difficulty in generalization, as the fabrication process should be accomplished under an inert air condition, so that it is difficult to control the process.
American U.S. Pat. Nos. 5,681,357, 5,688,293 and 5,834,135 disclose methods as follows: a solution in which polymer such as PvdF is dissolved in a solvent or in an organic solvent is applied to a polypropylene or polyethylene separation membrane used for the lithium ion battery. Then, the above membrane is dried and the dried membrane and the electrodes become thin layers by heat to be united. Thereafter, the organic electrolyte is inserted therein to fabricate the battery. The above method has a weak point that the polypropylene or polyethylene separation membrane may be deformed or pores of the separation membrane may be closed, as a polymeric solution of high concentration is cast in the polypropylene or polyethylene membrane. Further, the contact between the electrodes and the separation membrane are not sufficient, as the electrodes and the separation membrane are united by heating process to become thin layers, thereby causing a low high-rate charge/discharge property and a short cycle life. Thus, this method is hardly used.
Also American U.S. Pat. No. 5,853,916 discloses a process as follows to solve the problem: A cell is fabricated by coating a polyolefin porous separation membrane with polymer, which is followed by a gelling process at a predetermined temperature and pressure. However, the above method has a problem that a partial electrode position may be occurred at edges, because, to fabricate the battery in a wound form, a high pressure should be exerted on the battery in a vertical direction at a high temperature, so that a pressure in a horizontal direction is relatively lower. Further, the high-rate charge/discharge property of the battery and a cycle property are reduced, as a path of ion conduction is limited to the gel electrolyte. This method is not commercially used any more.